Nanomechanical memory demoed

By
Eric Smalley,
Technology Research NewsA
bit -- the basic unit of computer information -- can be made from anything
that can be switched between two states, which represent 1 and 0.

Computer chips use the presence and absence of electric current
to represent 1 and 0; disk drives use positive and negative magnetic poles.
The 19th- and early 20th-century precursors to today's computers used mechanical
rather than electrical elements to store and process data.

The rise of nanotechnology has led many researchers to revisit mechanical
computing. Nanotechnology has yielded microscopic materials that range from
microns, or thousandths of a millimeter -- around cell size -- to nanometers,
or millionths of a millimeter -- the realm of molecules. "It turns out that...
nanomechanical memory cells, due to their size and speed, could outperform
their counterparts in magnetoelectric systems," said Pritiraj Mohanty, an
assistant professor of physics at Boston University.

Several research teams have designed nanomechanical memory cells
based on carbon nanotubes or buckyballs that could lead to extraordinarily
fast, ultrahigh capacity computer memory. Carbon nanotubes are rolled up
sheets of carbon atoms that can be narrower than one nanometer; buckyballs
are spherical carbon molecules. The researchers have not found a way to
to mass-produce nanotube and buckyball memory devices, however.

A team at Boston University has made a minuscule mechanical memory
cell from silicon, the stuff of computer chips. The memory device is a beam
that is clamped at both ends. "If you take a metallic ribbon... and compress
it from both sides, it buckles into one of the two possible states: away
from you or towards you," said Mohanty. "Our idea was to use the two buckled
states as 0 and 1 for information storage," he said.

The memory cell beam is 8,000 nanometers long by 300 nanometers
wide by 200 nanometers high. A human hair, in contrast, is about 75,000
nanometers wide.

The device closely resembles silicon nanoscale oscillators that
researchers have been making for years and could be made by the millions
using standard chipmaking techniques. Nanoscale oscillators vibrate at high
speeds, and are being developed largely as compact, high-frequency sensors
and communications devices. The memory cell requires closer control than
oscillators in order to be switched between the two flexed states.

The memory cell beam flexes when current is sent through it, and
it can be switched between the two flexed states 23.5 million times per
second, or megahertz. Today's state-of-the-art memory chips operate at 400
megahertz. Shortening the beam to 1,000 nanometers will increase its frequency
to more than a billion times per second, or gigahertz, according to Mohanty.

The memory cell's size allows more than 100 gigabytes to be stored
per square inch, according to Mohanty. This is 125 times as much information
as today's memory chips hold. There are eight bits to a byte.

The device uses several orders of magnitude less power than today's
electronic memory. Its small range of motion means that it can be switched
using only femtowatts, or million billionths of a watt of power rather than
the microwatts, or millionths of a watt required to switch today's devices,
said Mohanty.

Mechanical memory is also resistant to radiation and electromagnetic
pulses, which can disrupt electronic and magnetic devices, said Mohanty.
This would make it useful for devices that operate under extreme conditions,
like spacecraft.

The challenge in making the minuscule memory beam was finding ways
to control and observe the its tiny movements, said Mohanty. "We spent a
lot of time... trying to talk to the structures," he said. "It is nontrivial
to [produce] the manipulation or read-write signal at high frequencies,
particularly when the structures are so small."

The researchers' method of monitoring the beam's motion is sensitive
enough to detect movements of less than one-tenth of a nanometer, according
to Mohanty. They found that the device moves four tenths of a nanometer
to go from a straight to a flexed position. The researchers are working
on methods to detect the smaller movements of shorter beams.

The researchers are also looking to make speedier memory using single-crystal
diamond rather than silicon, are investigating methods of making mechanical
memory retain data when the device is turned off, and are working out ways
to make the memory resistant to external disturbance, or noise, said Mohanty.

The memory could be used practically in two to five years, said
Mohanty. Several issues still need to be resolved, he said. "First, the
behavior and control of a large number of such memory cells, exceeding millions
per square inch, should be carefully studied. Second, manufacturing of these
cells and their integration with the current [chip] technology needs to
be explored."

Mohanty's research colleagues were Robert L. Badzey, Guiti Zolfagharkhani
and Alexei Gaidarzhy. The work appeared in the October 18, 2004 issue of
Applied Physics Letters. The research was funded by the National
Science Foundation (NSF), the Department of Defense (DOD) and the Sloan
Foundation.